Non-invasive blood pressure measurement device

ABSTRACT

A non-invasive blood pressure measurment device including: a cuff, a pressure detector for detecting a cuff pressure; a cuff pressure control pump for linearly increasing or decreasing the cuff pressure; a light-emitting member for injecting a beam of light into a part of a body by the cuff; light-receiving members for detecting an amount of light transmitted or an amount of light reflected of the beam of light injected into the body from the light-emitting member; a demodulating circuit for separating a pulsatile component from the light-receiving signal obtained from the light-receiving members; a CPU for sending a control signal to the cuff pressure control pump to thereby either increase the cuff pressure if it is judged that the pulsatile component has not been detected before applying pressure to the cuff based on the detection output from the demodulating circuit or decrease the once increased cuff pressure, and detecting an inflection point in the light-receiving signal in the course of increasing or decreasing the cuff pressure to thereby output a cuff pressure at the inflection point as a mean pressure value of a subject who is in systemic hypotension.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a non-invasive blood pressure measurmentdevice for measuring blood pressure of a subject in systemic hypotensionand relates to a non-invasive blood pressure measurement device formeasuring blood pressure of a subject by non-invasive means using avolume-oscillometric method. More particularly, the invention isdirected to an improved non-invasive blood pressure measurement devicethat can correct measurement errors attributable to vibration based onthe degree of variation in oxygen saturation measured simultaneouslywith the blood pressure.

2. Related Art

A photoelectric volume oscillometric method, for example, is known as aconventional method of measuring blood pressure of a subject bynon-invasive means.

The photoelectric volume oscillometric method is designed to measureblood pressure by detecting a feeble change in vibration caused by bloodpressure when the pressure of a cuff wrapped around a finger isincreased or decreased. That is, this method detects a change in thevolume of a blood vessel using a light-receiving element for receivingtransmitted light from a light-emitting element, and measures bloodpressure based on the cuff pressure and the amplitude of a photoelectricplethymograph signal that is an ac component of a light-receiving signaloutputted from the light-receiving element. FIG. 7 shows a relationshipbetween photoelectric cuff pressure and plethysmograph obtained when thecuff pressure is linearly increased. In the volume oscillometric method,a mean blood pressure value Pm can be calculated from a cuff pressurereading obtained when the plethysmograph has reached a maximum amplitudeL, and a systolic blood pressure can be measured from a cuff pressurereading recorded when the plethysmograph has reached, e.g., 20% of themaximum amplitude L by further increasing the cuff pressure. Also, adiastolic blood pressure Pd can be calculated from a cuff pressurereading at an inflection point of an envelope of the plethysmographobserved in the course of increasing the cuff pressure from 0 mmHg.

A detail of non-invasive blood pressue measurement using suchphotoelectric volume oscillometric method is described in a periodical,"Japanese Journal of Clinical Monitoring" (Vol. 13, No. 1, 1990, pp.75-84), published by the Clinical Monitor Society.

A non-invasive blood pressure measurement device that allows both bloodpressure measurement by the volume oscillometric method and oxygensaturation measurement to be made simultaneously is proposed in JapanesePatent Unexamined Publication No. 2-305555.

Since this conventional non-invasive blood pressure measurement devicebased on the volume oscillometric method is designed to measure bloodpressure by detecting a photoelectric volume pulse as described above,in the case where vibration is introduced from an external source when asubject is carried in an ambulance or in the case where noiseattributable to movement of his body is picked up, measurement errorshave resulted unavoidably.

By the way, when the subject is in shock due to ventricular fibrillationor in a low blood pressure state due to severe hemorage, theconventional non-invasive blood pressure measurement devices cannotprovide blood pressure measurements in some cases.

To overcome this problem, an attempt has been made to obtain some kindof blood pressure information from the subject in shock or the like.That is, what is proposed is a non-invasive blood pressure measurementcapable of determining that the subject is in systemic hypotension,which is less than 60 mmHg (or in pulsational arrest), by checking if aplethysmograph is detected while observing a plethysmograph signal fromthe light-receiving element under the cuff pressure set to apredetermined value, e.g., to 60 mmHg.

However, the blood pressure measurement under a cuff pressure of apredetermined value such as 60 mmHg permits a determination of thesystemic hypotension of a subject around such predetermined bloodpressure value. The blood pressure of a subject in shock cannot bemeasured by this method.

Amid demands for quick emergency medical treatment, it is of importancethat blood pressure be measured immediately after a subject in shock orthe like has been carried in the ambulance so that emergency medicaltreatment can be given him thereafter.

SUMMARY OF THE INVENTION

The invention has been made to overcome these problems encountered bythe conventional art. Accordingly, an object of the invention is toprovide a non-invasive blood pressure measurement device capable notonly of judging whether or not a subject is in systemic hypotension butalso of measuring the blood pressure of such subject in systemichypotension with ease.

Another object of the invention is to provide a non-invasive bloodpressure measurement device capable of measuring blood pressure bycontrolling errors even if noise due to vibration or movement of thebody is picked up.

To achieve the above objects, a first aspect of the invention is appliedto a non-invasive blood pressure measurement device, which includes: acuff for being attached to a part of a body of a subject; a pressuredetector for detecting a cuff pressure applied to the subject by thecuff; a pressure applying means for either applying pressure to the cuffby an inputted pressure increase control signal or dropping the cuffpressure by inputting a pressure decrease control signal; alight-emitting section for injecting a beam of light onto the part ofthe body to which pressure is applied by the cuff; a light-receivingsection for detecting an amount of light transmitted or an amount oflight reflected of the beam of light injected onto the body from thelight-emitting section; a pulse wave detecting means for separating apulsatile component from a light-receiving signal obtained from thelight-receiving section; a cuff pressure control means for outputtingthe pressure increase control signal to the pressure applying meanswhile receiving a detection output from the pressure detector when it isjudged that the pulsatile component has not been detected beforeapplying pressure to the cuff based on the detection output from thepulse wave detecting means, or outputting the pressure decrease controlsignal for dropping the once increased cuff pressure to the pressureapplying means; and a blood pressure value measuring means for detectingan inflection point of the light-receiving signal in the course ofincreasing the cuff pressure or in the course of decreasing the cuffpressure by the cuff pressure control means and outputting the cuffpressure at the inflection point as a mean blood pressure value of thesubject under the systemic hypotension in response to the detectionoutput from the pressure detector.

The non-invasive blood pressure measurement device according to theinvention is so designed that beams of light of a plurality ofwavelengths can be emitted from the light-emitting section on atime-sharing basis; and that when an inflection point is found in eitherone of the light-receiving signals of the plurality of wavelengths, acuff pressure at such inflection point is outputted as the mean bloodpressure value.

A second aspect of the invention is applied to a non-invasive bloodpressure measurement device, which includes: a cuff for being attachedto a part of a body of a subject; a pressure detector for detecting acuff pressure applied to the subject by the cuff; a pressure applyingmeans for applying pressure to the cuff by an inputted pressure increasecontrol signal, or dropping the cuff pressure by inputting a pressuredecrease control signal; a light-emitting section for injecting beams oflight of two different wavelengths, one being a beam of red light andthe other being a beam of infrared light, onto the part of the body towhich pressure is applied by the cuff; a light-receiving section fordetecting amounts of light transmitted or amounts of light reflected ofthe beams of light injected onto the body from the light-emittingsection; a signal component separating means for separating a dccomponent and a pulsatile component from light-receiving signals of therespective wavelengths obtained from the light-receiving section; anoxygen saturation calculating means for calculating a ratio of apulsation component in absorption of one wavelength due to arterialblood flow to that of the other wavelength from the dc components andthe pulsatile components of the respective wavelengths obtained from thesignal component separating means, and calculating an oxygen saturationfrom the ratio; a permissible range of variance calculating means forobtaining a mean value of oxygen saturations of the subject beforeapplying pressure to the cuff, and calculating a permissible range ofvariance from the mean value, the oxygen saturations being thoseobtained by the oxygen saturation calculating means; a cuff pressurecontrol means for outputting the pressure increase control signal to thepressure applying means while receiving the detection output from thepressure detector, or outputting the pressure decease control signal fordropping the once increased cuff pressure to the pressure applying meansafter the permissible range of variance has been obtained; a judgingmeans for judging whether or not an oxygen saturation obtained in thecourse of increasing or decreasing the cuff pressure is within thepermissible range of variance; and a blood pressure measuring means forselecting a valid pulse wave signal from the pulse wave signals obtainedby the signal component separating means in the course of increasing ordecreasing the cuff pressure based on a result of the judgment by thejudging means, and calculating a blood pressure value of the subject bya volume oscillometric method from an amplitude of the valid pulse wavesignal and the cuff pressure value obtained by the pressure detector.

As described above, according to the first aspect of the invention, asubject in shock or exhibiting an extremely low blood pressure, for whomit is difficult to measure blood pressure by the conventional volumeoscillometric method, can be subjected not only to judgment of whetheror not he is in the systemic hypotension, but also to measure the bloodpressue of the subject in the systemic hypotension. Therefore, theinvention can make a great contribution to the improvement of clinicaltreatment, particularly, of emergency medical treatment.

Moreover, according to the second aspect of the invention, in ameasuring environment involving vibration or movement of the body forwhich it is difficult to measure blood pressure by the conventionalvolume oscillometric method, reliable blood pressure measurement can beeffected on a subject.

Accordingly, the invention can improve reliability of blood pressuremeasurment in ambulances, which have measurement heretofore imposedproblems, thereby contributing to the improvement of emergency medicaltreatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a non-invasive blood pressuremeasurement device, which is a first embodiment of the invention;

FIG. 2 is a flowchart showing an operation procedure of thesphygmomanometer of FIG. 1;

FIG. 3(a) is a schematic diagram of a tissue in the case of measuringblood pressure by detecting transmitted light;

FIG. 3(b) is a diagram showing increase in a cuff pressure duringmeasurement;

FIG. 3(c) is a diagram showing luminous intensity of a light-receivingsignal detected when such increase has occurred;

FIG. 4 is a diagram illustrative of how reflected light after itstransmission through the tissue is received by a light-receivingsection;

FIG. 5(a) is a schematic diagram showing the tissue in the case ofmeasuring blood pressure by detecting reflected light;

FIG. 5(b) is a diagram showing increase in the cuff pressure duringmeasurement;

FIG. 5(c) is a diagram showing luminous intensity of a light-receivingsignal detected when such increase has occurred;

FIG. 6 is a characteristic diagram illustrative of an example in whichabsorption of tissue, arterial blood, and venous blood coincide with oneanother, by wavelength;

FIG. 7 is a diagram showing a relationship between cuff pressure andamplitude of a plethysmograph in a volume oscillometric method;

FIG. 8 is a block diagram showing a non-invasive blood pressuremeasurement device, which is a second embodiment of the invention;

FIG. 9 is a flowchart showing an operation procedure of thesphygmomanometer of FIG. 8;

FIG. 10(a) is a schematic diagram of the tissue in the case of measuringblood pressure by detecting transmitted light;

FIG. 10(b) is a diagram showing increase in the cuff pressure duringmeasurement;

FIG. 10(c) is a diagram showing a waveform of received light of awavelength λ1, the waveform being detected when such increase hasoccurred;

FIG. 10(d) is a diagram showing a waveform of the received light of awavelength λ2, the waveform being detected when such increase has takenplace;

FIG. 11(a) is a diagram showing a waveform of received light of thewavelength λ1 when noise has been picked up;

FIG. 11(b) is a diagram showing a waveform of the received light of thewavelength λ2 when noise has been picked up;

FIG. 11(c) is a diagram showing variation in oxygen saturation whennoise has been picked up;

FIG. 12 is a diagram illustrative of how reflected light after itstransmission through the tissue is received by the light-receivingsection;

FIG. 13(a) is a schematic diagram of the tissue in the case of measuringblood pressure by detecting reflected light; and

FIG. 13(b) is a diagram showing increase in the cuff pressure duringmeasurement;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basic concept of a first embodiment of the invention will now bedescribed.

It can be determined that a subject is in systemic hypotension pressureif no pulsatile component is found in a measured signal from the subjectbefore applying pressure to a cuff.

When pressure is applied to a part of the body of a subject who is insystemic hypotension, e.g., to a finger of his hand, by a cuff 4 thatincreases the cuff pressure linearly as shown in FIG. 3(b), thethickness of a tissue portion 7a of a tissue 7 is maintained constant,whereas arterial blood 7b and venous blood 7c thereof are graduallyeliminated as shown in FIG. 3(a). The tissue 7 is interposed between alight-emitting section 5 and a light-receiving section 6A (see FIG. 1),and the tissue portion 7a does not contain blood. As a result, theluminous intensity of transmitted light detected at the light-emittingsection 6A gradually increases due to a decrease in light-extinguishingelements (the arterial blood and the venous blood) associated with anincrease in the cuff pressure, making the cuff pressure larger than thearterial blood pressure. When all the blood has been eliminated, leavingonly the tissue portion 7a, change in the luminous intensity settles ata substantially constant level. At a timing when all the arterial bloodhas been eliminated completely, a drastic change is observed in theinclination of the luminous intensity as shown in FIG. 3(c). If suchtiming is detected as an inflection point a and if the cuff pressure isread at this inflection point a, then a mean blood pressure value of thesubject who is in systemic hypotension can be measured.

On the other hand, if the cuff 4 is attached to a brachial arm of thesubject, the transmitted light cannot be detected. Therefore, thelight-emitting section 5 and a light-receiving section 6B are arrangedon the same plane so as to be equidistantly apart from each other, sothat reflected light is detected by the light-receiving section 6B aftera beam of light emitted from the light-emitting section 5 has beenextinguished by its transmission through the tissue 7.

In this case, if the cuff pressure is linearly increased as shown inFIG. 5(b) after the subject has been judged to be in systemichypotension, then an optical path length D, which is a distance betweenthe light-emitting section 5 and the light-receiving section 6B can bemaintained constant. However, since the arterial blood 7b and the venousblood 7c are being gradually eliminated, the luminous intensity of thereflected light detected by the light-receiving section 6B graduallyincreases. When the cuff pressure has exceeded the arterial bloodpressure, all the blood is removed to leave only the tissue portion 7a.As a result, the inclination of the luminous intensity of the reflectedlight changes drastically as shown in FIG. 5(c), and the change withrespect to the cuff pressure becomes extremely small thereafter. If aninflection point a is detected and if the cuff pressure is read at suchpoint, a mean blood pressure value of the subject who is in systemichypotension can be measured.

By the way, there exists a wavelength at which all the tissue portion 7anot containing blood, the arterial blood 7b, and the venous blood 7cexhibit the same absorption characteristic. FIG. 6 shows an example inwhich an absorption characteristic G3 of the tissue portion, anabsorption characteristic G1 of the arterial blood, and an absorptioncharacteristic G2 of the venous blood coincide with one another at theabsorption wavelength λ2, out of three wavelengths λ1, λ2, λ3. Bloodpressure measurement using a wavelength at which the absorption of thetissue portion, the arterial blood, and the venous blood coincide withone another is difficult, especially when the measurement involves lightreception of the reflected light, because the optical path length isinconstant regardless of the fact that the arterial blood and the venousblood are eliminated from the tissue.

To overcome this problem, if it is so arranged that beams of light of aplurality of wavelengths, e.g., three wavelengths, can be sequentiallyemitted from the light-emitting section 5, there is no likelihood thatthe absorption characteristics of all the wavelengths will coincide withone another. Therefore, such arrangement allows not only judgment ofwhether or not a subject is in systemic hypotension but also, if it hasbeen determined that the subject is under the systemic hypotension,measurement of a mean blood pressure value using any one of theplurality of wavelengths.

Further, in measurements involving the detection of transmitted light,the use of a plurality of wavelengths ensures reliable and accuratemeasurement.

A non-invasive blood pressure measurement device, which is a firstembodiment of the invention, will now be described in detail withreference to the accompanying drawings.

The sphygmomanometer, which is the first embodiment, is shown by a blockdiagram in FIG. 1. In FIG. 1, a light-emitting section 5 and alight-receiving section 6A are mounted on the inner surface of a cuff 4so as to be integral with the cuff 4, which is attached to a part of thebody of a subject, e.g., a finger of his hand. The light-emittingsection 5 is used to inject beams of light onto a tissue 7 of thesubject, and the light-receiving section 6A is disposed on a sideopposite to the light-emitting section 5 so that the tissue 7 isinterposed therebetween. Here, it is assumed that the tissue 7 includes:a tissue portion 7a not containing blood; arterial blood 7b, and venousblood 7c (see FIG. 3(a)). The light-receiving section 5 is formed oflight-emitting elements that emit beams of light of three differentwavelengths, such as three light-emitting diodes 5a, 5b, 5c. Emittedfrom the respective light emitting diodes 5a, 5b, 5c are a beam of lightwith a first wavelength of 660 nm, a beam of light with a secondwavelength of 805 nm, and a beam of light with a third wavelength of 940nm. Instead of using the three light-emitting diodes, filters of therespective colors disposed in front of light sources may be switched toemit beams of light of three wavelengths. The light-receiving section 6Ais formed of a light-receiving element such as a phototransistor, anddetects the amounts of light transmitted after the beams of lightinjected from the light-emitting section 5 have been transmitted throughthe tissue 7.

The cuff 4 is connected to a cuff pressure control pump 8 through an airtube 8a, the pump 8 including a drive circuit. The operation ofincreasing, decreasing, or releasing the cuff pressure can be performedby inputting a control signal from a central processing unit (CPU) 1 tothe pump 8 that serves as a pressure applying means.

The pressure to be applied to the tissue 7 by the cuff 4 is detected bya pressure detector 9, and the output of the pressure detector 9 isreceived by the CPU 1 after being converted into a digital signal by anA/D converter 12.

A timing generating circuit 2 under control of the CPU 1 applies notonly pulse signals to buffers 3a, 3b, 3c of a driver circuit 3, but alsotiming signals to a demodulating circuit 11. The pulse signals determinetimings for sequentially driving the respective light-emitting diodes5a, 5b, 5c on a time-sharing basis. The timing signals serve to separatelight-receiving signals from the output signals of the light-receivingsection 6A at every wavelength.

The respective buffers 3a, 3b, 3c sequentially amplify the receivedpulse signals and drive the respective light-emitting diodes 5a, 5b, 5c.Accordingly, the light-emitting diodes 5a, 5b, 5c sequentially injectbeams of light with the first into the third wavelengths to the tissue7.

The light-receiving section 6A detects the amounts of light transmittedafter the beams of light injected from the light-emitting diodes 5a, 5b,5c have been extinguished by their transmission through the tissue 7.These light-receiving signals are supplied to the demodulating circuit11 after being amplified by an input amplifier 10.

The demodulating circuit 11 not only separates light-receiving signalsat every wavelength from its input signals based on the timing signals,but also separates a dc component and a pulsatile component from eachlight-receiving signal. The signal components outputted from thedemodulating circuit 11 are received by the CPU 1 after being convertedinto digital signals.

Here, the demodulating circuit 11 constitutes a pulse wave detectingmeans, and the CPU 1 constitutes a cuff pressure controlling means and ablood pressure measuring means.

An operation of the thus constructed non-invasive blood pressuremeasurement device will be described with reference to a flowchart ofFIG. 2.

At the measurement start timing, the cuff is not pressured; i.e., thecuff pressure is maintained at 0 mmHg (Step S1). Under this condition,the beams of light of the respective wavelengths are sequentiallyinjected onto the tissue 7 from the light-emitting diodes 5a, 5b, 5c ofthe light-emitting section 5, and the amounts of light transmitted aredetected by the light-receiving section 6A. From each signal outputtedfrom the light-receiving section 6A are a dc component and a pulsatilecomponent separated in the demodulating circuit 11, and these componentsare received by the CPU 1 (Step S2). The CPU 1 determines whether or noteach light-receiving signal includes the pulsatile component (Step S3).If the pulsatile component has been detected, the CPU 1 performs aseries of processing steps for measuring a mean blood pressure value Pm,a systolic blood pressure value Ps, and a diastolic blood pressure valuePd of the subject using photoelectric volume oscillometric method (StepS4).

On the other hand, if no pulsatile component has been detected, it isjudged that the subject is in systemic hypotension. The CPU 1 outputs apressure increase control signal to the cuff pressure control pump 8.Accordingly, the pressure of the cuff 4 is linearly increased by thepump 8 (Step S5). In the course of increasing the cuff pressure, the CPU1 sequentially measures the luminous intensities of the transmittedlight beams of the three wavelengths, and calculates differences withrespect to the luminous intensities before the cuff pressure has beenincreased (Steps S6 and S7). The CPU 1 then determines whether or not aninflection point a at which the inclination of the luminous intensity ofthe transmitted light beam chagas drastically exists for any of thethree wavelengths based on the calculated values. If no inflection pointhas been found, then the CPU 1 determines whether or not the cuffpressure has reached a set maximum value, e.g., 180 mmHg. If the maximumcuff pressure has not been reached, the processing is repeated from StepS5.

If the inflection point a has been found in Step S8 for any of the threewavelengths, then the cuff pressure at such inflection point a isdetected and displayed as a mean blood pressure value (Step S10). At thesame time, a cuff pressure release signal is sent to the cuff pressurecontrol pump 8 to release the cuff pressure (Step S11).

If it is judged that the maximum cuff pressure has been reached in StepS9, then either the cuff pressure is measured again after releasing thecuff pressure or the processing to be performed when measurement is notpossible is performed (Steps S12, S13).

More accurate measurement can be made if the processing for measuringthe mean blood pressure value is performed again by sending a pressuredecrease control signal from the CPU 1 to the cuff pressure control pump8 and detecting the inflection point a even in the course of linearlydecreasing the cuff pressure after the cuff pressure has reached themaximum blood pressure.

Then, a case where measurement is made by attaching the cuff 4 to abrachial arm of the subject will be described. In this case, thelight-emitting section 5 and a light-receiving section 6B are linearlymounted on the inner surface of the cuff 4 so as to be apart from eachother by a predetermined distance as shown by a broken line in FIG. 1.The predetermined distance is the optical path length D between thelight-emitting section 5 and the light-receiving section 6B.

In this case, the respective light-emitting diodes 5a, 5b, 5c of thelight-emitting section 5 sequentially inject beams of light with threedifferent wavelengths into the tissue 7. The amounts of light reflectedafter the beams of light have been extinguished by their transmissionthrough the tissue 7 are detected by the light-receiving section 6B. Ameasurement procedure in this case is as shown by the flowchart of FIG.2.

While the light-emitting section 5 is formed of the three light-emittingdiodes 5a, 5b, 5c that emit beams of light of different wavelengths inthe above two cases, the number of wavelengths of light emitted from thelight-emitting section 5 are not limited to three; e.g., the number ofwavelengths may be two.

Particularly, when the measurement is made on a finger of a hand, it isthe transmitted light, not the reflected light, that is detected.Therefore, the arrangement in which a beam of a single wavelength isemitted by the light-emitting section 5 may be acceptable.

As described above, according to the first embodiment of the invention,a subject in shock or exhibiting extremely low blood pressure, for whomit is difficult to measure blood pressure by the conventional volumeoscillometric method, can be analyzed not only as to whether or not heis in the systemic hypotension, but also as to his actual blood pressueeven if he is in systemic hypotension. Therefore, the invention can makea great contribution to the improvement of clinical treatment,particularly, of emergency medical treatment.

Second embodiment

The basic concept of a second embodiment of the invention will bedescribed next. The parts which have been described with reference tothe first embodiment are designated by the same reference numerals orcharcters.

In the case where blood pressure is measured by wrapping the cuff 4around a part of the body of a subject, e.g., a finger of his hand, thetissue 7 interposed between the light-emitting section 5 and thelight-receiving section 6A (see FIG. 8) can be schematically illustratedas shown in FIG. 10(a). Here, reference character 7a designates a tissueportion not containing blood; 7b, arterial blood that pulsates; and 7c,venous blood.

When two wavelengths λ1, λ2 of red light and infrared light are injectedfrom the light-emitting section 5 on a time-sharing basis, thelight-receiving section 6A produces transmitted light outputs ofwavelengths λ1, λ2 such as shown in FIGS. 10(c), (d). The luminousintensities of the transmitted light beams with these wavelengths aredetected as a pulsatile component (ac component) ΔI₁.ΔI₂ superposed on adc component I_(1DC).I_(2DC). A pulsation component ΔA1 in theabsorption of the wavelength λ1 of the red light and a pulsationcomponent ΔA2 in the absorption of the wavelength λ2 of the infraredlight can be calculated from the following equations.

    ΔA1=ΔI.sub.1 /I.sub.1DC

    ΔA2=ΔI.sub.2 /I.sub.2DC

A ratio Φ of the pulsation component in the absorption of the wavelengthλ1 to the pulsation component in the absorption of the wavelength λ2 canbe given by the following equation, using ΔA1, ΔA2.

    Φ=ΔA1/ΔA2

The oxygen saturation S can be given as a function f of the absorptionratio Φ.

An oxygen saturation calculating means implemented by the CPU 1 measuresthe oxygen saturation S in a first measuring area, i.e., before the cuffpressure is applied, or when the cuff pressure is 0 mmHg. If noise dueto vibration or movement of the body is introduced into the subject,then outputs of the transmitted light including a noise component N suchas shown in FIGS. 11(a), (b) are detected at the light-receiving section6A. Therefore, the oxygen saturations S measured when the noisecomponent has been picked up (indicated by crosses) exhibit variationcompared with those measured when the subject is at rest (indicated bycircles) as shown in FIG. 11(c).

A permissible range of variance calculating means implemented by the CPU1 calculates a mean value K of the oxygen saturations S havingvariations measured in the first measuring area, and multiplies K by anallowance (±A%), which is an appropriate coefficient A, to obtain apermissible range of variance δ. Here, the allowance is a valuepredetermined by a measurement environment in which the subject isplaced, the measurement environment being, e.g., vibration the subjectreceives (e.g.) vibration received by the subject when the subject iscarried in the ambulance). Any arbitrary value can be inputted to theCPU 1 from an external unit as the allowance.

Then, when the cuff pressure is increased linearly as shown in FIG.10(b) by the pressure applying means that is under control of the cuffpressure control means implemented by the CPU 1, the finger of the handof the subject is pressed by the cuff 4, which in turn graduallyeliminates both the arterial blood 7b and the venous blood 7c that serveas the elements for extinguishing the light (see FIG. 10(a)). As aresult, the luminous intensities of the transmitted light beams detectedat the light-receiving section 6A are gradually increased. This is asecond measuring area.

When the cuff pressure is further increased so as to exceed the arterialblood pressure, all the blood is eliminated, leaving only the tissueportion 7a. Thus, the luminous intensities of the light-receivingoutputs change to an extremely small degree. This is a third measuringarea.

If the oxygen saturations S in the second and the third measuring areasare measured by the oxygen saturation calculating means beat by beat, ajudging means implemented by the CPU 1 determines whether or not themeasured result is within the permissible range variance δ. Actually, adifference between the measured oxygen saturation S and the mean value Kis calculated, and whether or not the calculated result is within thepermissible range of variance δ is checked. By defining a sum of themean value K and the permissible range of variance δ as a permissiblerange of variance δ', whether or not the measured oxygen saturation S iswithin the permissible range of variance δ' may be determined.

A signal component separating means separates a photoelectricplethysmograph of at least one of the wavelengths of the light-receivingsignals obtained in the second and the third measuring areas. A bloodpressure measuring means implemented by the CPU 1 measures the amplitudeof the photoelectric plethysmograph, and if the judging means determinesthat the measured oxygen saturation S is within the permissible range ofvariance δ, then the mean blood pressure value Pm, the systolic bloodpressure value Ps, and the diastolic blood pressure value Pd aremeasured from the amplitude of the photoelectric plethysmographdetermined as being valid and the cuff pressure value. If the measuredoxygen saturation S is determined to be out of the permissible range ofvariance δ, then the processing for treating the separated photoelectricplethysmograph as being invalid is performed instantly, thereby notallowing the CPU 1 to calculate the blood pressure value using suchinvalid photoelectric plethysmograph.

The blood pressure measurement based on the volume oscillometric methodeffected by the above procedure can invalidate measurements obtainedwhen the subject is under sever vibration in the second measuring areaor measurements obtained in the third measuring area in which the accomponent consisting of external noise due to, e.g., vibrationerroneously recognized as a pulse wave that has not been detected isdetected. Therefore, more reliable measurement is possible.

On the other hand, when the cuff 4 is attached to a brachial arm of thesubject, the transmitted light cannot be detected. Therefore, with thelight-emitting section 5 and a light-receiving section 6B disposedequidistantly apart from each other on the same plane, reflected lightafter light beams emitted from the light-emitting section 5 have beenextinguished by their transmission through the tissue 7 is detected atthe light-receiving section 6B.

In this case, if the cuff pressure is linearly increased as shown inFIGS. 13(a), (b), then the optical path length D that corresponds to thedistance between the light-emitting section 5 and the light-receivingsection 6B is maintained constant. However, since the arterial blood 7bincluding pulsation and the venous blood 7c are being graduallyeliminated, the luminous intensities of the reflected light beamsdetected at the light-receiving section 6B are on the gradual increaseas the detected waveforms shown in FIGS. 10(c), (d) and FIGS. 11(a),(b). When the cuff pressure has exceeded the arterial blood pressure,all the blood is eliminated to leave only the tissue portion 7a. As aresult, the luminous intensities of the reflected light beams change toan extremely small degree.

Thus, even in the method involving the detection of the reflected light,reliable blood pressure measurement can be made by following the aboveoperation procedure.

A sphygmomanometer, which is the second embodiment of the invention,will be described in detail with reference to the accompanying drawings.

The sphygmomanometer, which is the second embodiment, is shown in ablock diagram of FIG. 8. In FIG. 8, the light-emitting section 5 and thelight-receiving section 6A are mounted integrally with one another onthe inner surface of the cuff 4 that is attached to a part of the bodyof a subject, e.g., a finger of his hand. The light-emitting section 5is used to inject beams of light into the tissue 7 of the subject, andthe light-receiving section 6A is disposed on a side opposite to thelight-emitting section 5 so that the tissue 7 can be interposedtherebetween. Here, it is assumed that the tissue 7 includes: the tissueportion 7a not containing blood; the arterial blood 7b, and the venousblood 7c (see FIG. 10 (a)). The light-receiving section 5 is formed oflight-emitting elements for emitting beams of light of two differentwavelengths, such as two light-emitting diodes 5a, 5b. Emitted from therespective light emitting diodes 5a, 5b are a beam of red light with afirst wavelength λ1 of 660 nm and a beam of the infrared light with asecond wavelength λ2 of 940 nm. Instead of using the two light-emittingdidoes, filters of the respective colors disposed in front of lightsources may be switched to emit the beams of light of two wavelengths.The light-receiving section 6A is formed of a light-receiving elementsuch as a phototransistor, and detects the amounts of light transmittedafter the light beams injected from the light-emitting section 5 havebeen transmitted through the tissue 7.

The cuff 4 is connected to a cuff pressure control pump 8 through an airtube 8a, the pump 8 including a drive circuit. The operation ofincreasing, decreasing, or releasing the cuff pressure can be performedby applying a control signal from the CPU 1 to the pump 8 that serves asa pressure applying means.

The cuff pressure to be applied to the tissue 7 through the cuff 4 isdetected by a pressure detector 9, and the output of the pressuredetector 9 is received by the CPU 1 after being converted into a digitalsignal at an A/D converter 12.

A timing generating circuit 2 under control of the CPU 1 applies notonly pulse signals to buffers 3a, 3b of a driver circuit 3, but alsotiming signals to a demodulating circuit 11. The pulse signals determinetimings for sequentially driving the respective light-emitting diodes5a, 5b on a time-sharing basis. The timing signals serve to separatelight-receiving signals from output signals of the light-receivingsection 6A by the respective wavelengths λ1, λ2.

The respective buffers 3a, 3b sequentially amplify the received timingpulse signals and drive the respective light-emitting diodes 5a, 5b.Accordingly, the light-emitting diodes 5a, 5b sequentially inject beamsof light with the first and the second wavelengths λ1, λ2 to the tissue7.

The light-receiving section 6A detects the amounts of light transmittedafter the beams of light injected from the light-emitting diodes 5a, 5bhave been extinguished by their transmission through the tissue 7. Theselight-receiving signals are supplied to the demodulating circuit 11after amplified by an input being amplifier 10.

The demodulating circuit 11 serving as a signal component separatingmeans not only separates the light-receiving signals by the respectivewavelengths λ, λ2 from its input signals based on the timing signals,but also separates a dc component and a pulsatile component from eachlight-receiving signal. The respective signal components outputted fromthe demodulating circuit 11 are received by the CPU 1 after beingconverted into digital signals.

An allowance (±A%), which is a coefficient used for calculating thepermissible range of variance δ from the mean value K of the oxygensaturations S measured before applying pressure to the cuff can be inputto the CPU 1 from an external unit.

An operation of the thus constructed non-invasive blood pressuremeasurment device will be described next with reference to a flowchartof FIG. 9.

At the measurement start timing, the cuff is not pressured; i.e., thecuff pressure is maintained at 0 mmHg (Step S1). Under this condition,beams of light of the respective wavelengths λ1, λ2 are sequentiallyinjected into the tissue 7 from the light-emitting diodes 5a, 5b of thelight-emitting section 5, and the amounts of light transmitted aredetected at the light-receiving section 6A. From each signal outputtedfrom the light-receiving section 6A a dc component are and a pulsatilecomponent separated, and these components are received by the CPU 1(Step S2). The CPU 1 processes the input signals based on theabove-mentioned operation procedure to measure the oxygen saturation Sbeat by beat, and calculate a mean value K of the measured oxygensaturations S (Step S3). The CPU 1 further calculates the permissblerange of variance δ by multiplying the mean value K by the allowance(±A%) (Step S4).

Then, the CPU 1 outputs a pressure increase control signal to the cuffpressure control pump 8. Accordingly, the pressure of the cuff 4 islinearly increased by the pump 8 (Step S5). In the course of increasingthe cuff pressure, the operation similar to Step S2 is performed, sothat the dc components and the pulsatile components separated from thelight-receiving signals by the respective wave lengths λ1, λ2 arereceived by the CPU 1 (Step S6). The CPU 1 measures not only theamplitudes of the detected pulsatile components, i.e., photoelectricplethsmograph signals, but also the oxygen saturations S (Steps S7 andS8).

Further, the CPU 1 calculates a difference between the measured oxygensaturation S and the mean value K obtained in Step S3, and determineswhether or not such difference is within the permissble range ofvariance δ (Steps S9 and S10). If the difference is judged to be withinthe permissble range of variance δ, Step S11 will be executed. On theother hand, if the difference is determined to be out of the permissblerange of variance δ, due to noise caused by vibration or movement of thebody, processing branches to step S12, for invalidating the detectedphotoelectric plethysmograph signal immediately.

Whether or not the cuff pressure has reached a set maximum value, e.g.,180 mmHg is determined in Step S13. If the maximum cuff pressure has notbeen reached, the processing is repeated from Step S5.

By a series of processing steps, the mean blood pressure value Pm, thesystolic blood pressure value Ps, and the diastolic blood pressure valuePd of the subject are calculated from the amplitude of the validphotoelectric plethysmograph signal and the cuff pressure value everypulse in Step S11.

If it is determined that the cuff pressure has reached the maximum bloodpressure in Step S13, then the cuff pressure is released to terminatethe measurement (Step S14).

More reliable measurement may be achieved if blood pressure is measuredagain in the course of linearly decreasing the cuff pressure by sendinga pressure decrease control signal to the cuff pressure control pump 8after the cuff pressure has reached the maximum blood pressure.

By measuring blood pressure based on the volume oscillometric methodwhile performing the above-mentioned processing, the sphygmomanometer ofthe invention can eliminate measurements in the third measuring area(see FIG. 11(c)) in which the measured oxygen saturation S differsmarkedly from the correct value as a result of erroneously recognizing anon-pulse noise component as a pulse, the noise component being causedby vibration or the like. The sphygmomanometer of the invention can alsoeliminate a measured value if such measured value is obtained from alarge noise component out of the allowable range having been temporarilypicked up in the second measuring area in which a pulse is detected; theS/N ratio is relatively large; and the oxygen saturations S are lesserratic. Therefore, highly reliable measurement is possible.

Now, an embodiment in which measurement is made by attaching the cuff 4to a brachial arm of the subject will be described. In this embodiment,the light-emitting section 5 and the light-receiving section 6B aremounted on the inner surface of the cuff 4 so as to be separated fromeach other by a predetermined distance as shown by a broken line in FIG.8. The predetermined distance is the optical path length D between thelight-emitting section 5 and the light-receiving section 6B.

In this case, the respective light-emitting diodes 5a, 5b of thelight-emitting section 5 sequentially inject beams of light of twowavelengths λ1, λ2 into the tissue 7. The amounts of light reflectedafter the beams of light have been extinguished by their transmissionthrough the tissue 7 are detected by the light-receiving section 6B. Ameasurement procedure in this case is as shown by a flowchart of FIG. 2.

As described above, according to the second embodiment of the invention,in a measuring environment involving vibration or movement of the bodyfor which it is difficult to measure blood pressure by the conventionalvolume oscillometric method, reliable blood pressure measurement can beeffected on a subject.

Accordingly, the invention can improve reliability of blood pressuremeasurement in ambulances, which have heretofore imposed problems,thereby contributing to the improvement of emergency medical treatment.

What is claimed is:
 1. A non-invasive blood pressure measurement devicecomprising:a cuff configured to be attached to a body part of a subject;a pressure detector operatively connected to said cuff for detecting acuff pressure applied to the body part by the cuff and outputting adetection output; pressure applying means for applying pressure to thecuff in response to an input pressure increase control signal and fordropping the cuff pressure in response to an input pressure decreasecontrol signal; a light-emitting member for injecting beams of light oftwo different wavelengths, one being a beam of red light and the otherbeing a beam of infrared light, into the body part of the subject towhich pressure is applied by said cuff; a light-receiving member fordetecting one of amounts of light transmitted and amounts of lightreflected of the beams of light injected into the body part from saidlight-emitting member, and thereby outputting light-receiving signalsfor the two different wavelengths; signal component separating meansoperatively connected to said light-receiving member for separating a dccomponent and a pulsatile component from each of the light-receivingsignals of the respective wavelengths obtained from said light-receivingmember; oxygen saturation calculating means for calculating a ratio of apulsation component in absorption of one wavelength due to arterialblood flow to that of the other wavelength from the dc components andthe pulsatile components of the respective wavelengths obtained fromsaid signal component separating means, and for calculating an oxygensaturation from the ratio; permissible range of variance calculatingmeans for obtaining a mean value of oxygen saturation measurements ofthe subject before said pressure applying means applies pressure to thecuff, and for calculating a permissible range of variance from the meanvalue, the oxygen saturation measurements being obtained from saidoxygen saturation calculating means; cuff pressure control means foroutputting the pressure increase control signal to said pressureapplying means while receiving the detection output from said pressuredetector, and for outputting the pressure decrease control signals, fordropping the once increased cuff pressure, to said pressure applyingmeans, after the permissible range of variance has been obtained;determining means for determining whether or not an oxygen saturationobtained in the course of increasing or decreasing the cuff pressure iswithin the permissible range of variance and for outputting adetermination result; and blood pressure measuring means for selecting,based on the determination result output by said determining means, avalid pulse wave signal from the pulsatile components obtained by saidsignal component separating means in the course of increasing ordecreasing the cuff pressure and for calculating a blood pressure valueof the subject by a volume oscillometric method from an amplitude of thevalid pulse wave signal and the detection output obtained from saidpressure detector.
 2. A method for measuring blood pressurenon-invasively, the method comprising the steps of:injecting beams oflight of two different wavelengths, one being a beam of red light andthe other being a beam of infrared light, into a body part of a subjectto which a cuff is attached; detecting one of amounts of lighttransmitted and amounts of light reflected of the beams of lightinjected into the body part and outputting, for each of the twowavelengths, a resultant light-receiving signal; separating a dccomponent and a pulsatile component from each of the light-receivingsignals of the respective wavelengths; calculating a ratio of apulsation component in absorption of one wavelength due to arterialblood flow to that of the other wavelength from the dc components andthe pulsatile components of the respective wavelengths; calculating anoxygen saturation from the ratio; obtaining a mean value of oxygensaturation measurement of the subject before applying pressure to thecuff; calculating a permissible range of variance from the mean value;increasing pressure to the cuff while detecting the cuff pressure, ordecreasing the cuff pressure while detecting the cuff pressure, afterthe permissible range of variance is calculated; determining whether ornot an oxygen saturation obtained in the course of increasing ordecreasing the cuff pressure is within the permissible range of varianceand then outputting a determination result; and selecting, based on thedetermination result, a valid pulse wave signal from the pulsatilecomponents in the course of increasing or decreasing the cuff pressure;calculating a blood pressure value of the subject by a volumeoscillometric method from an amplitude of the valid pulse wave signaland the detected cuff pressure.